The present invention relates to a process for producing pyromellitic dianhydride from 1,2,4,5-tetramethylbenzene(durene) by vapor-phase catalytic oxidation with a gas containing molecular oxygen. The present invention also relates to a catalyst used in this process.
Pyromellitic dianhydride (PMDA) has been used extensively as raw material for heat-resistant resins or as a plasticizer or a curing agent for epoxy resins. More recently, the potential for producing durene at a lower cost using an alkylation catalyst such as zeolite has increased and there is a growing interest in the importance of PMDA as an industrial raw material.
PMDA can be synthesized by either liquid-phase oxidation(oxidation with nitric acid or a cobalt acetate-sodium bromide system) or vapor-phase catalytic oxidation using a catalyst. For small-scale production, liquid-phase oxidation is suitable and is the principal commercial method used today. However, in view of the growing demand for PMDA, vapor-phase catalytic oxidation which is suitable for large-scale production is anticipated to become a predominant commercial process to be adopted in the future.
In PMDA production from durene by vapor-phase catalytic oxidation, a catalytic component based vanadium pentoxide is carried on a support such as fused alumina (.alpha.-alumina) or silicon carbide (these materials have a specific surface area of no larger than 1 m.sup.2 /g) and is used as a catalyst. A catalyst solely composed of vanadium pentoxide on a support may be used in PMDA production but suffers from various disadvantages such as formation of by-products in large quantities, low conversion and low yield of PMDA. To avoid these problems, it is a common practice to use a catalyst that is based on vanadium pentoxide which, in combination with additional catalytic components selected from oxides (chiefly metal oxides), is carried on a support. Examples of this catalyst are described in many patents such as Japanese Patent Publication Nos. 42-1008, 42-15925, 43-26497, 45-4978, 45-15018, 45-15252, 46-14332, 49-31972 and 49-31973. However, if these prior art catalysts are used in a fixed-bed reactor which is adopted customarily in industrial catalytic reactions, the high sensitivity of the reaction to temperature causes the following problems. In commercial production of PMDA, a reaction tube typically having a diameter of 1 inch is filled with ca. 0.5-2 liters of a catalyst and submerged in a molten salt bath, with durene supplied into the reaction tube from the top together with a molecular oxygen containing gas(normally air). In the catalyst bed, molecular oxygen reacts with durene by the catalytic action to produce PMDA. At the same time, part of the durene feed undergoes excessive reaction and is completely oxidized to evolve gases such as carbon dioxide and carbon monoxide.
The catalysts disclosed in the above-listed prior patents have high reactivity and may produce PMDA with high selectivity but even in this case, the conversion to gases (i.e. CO.sub.2 and CO) is high and cannot be reduced to 35% or below. Furthermore, the prior art catalyst systems have great temperature dependency and the conversion to oxidized gases increases rapidly if the reaction temperature is outside an optimal range. To make the case worse, the optimal range of reaction temperature is very narrow (20.degree.-30.degree. C.).
When the catalyst is packed in the reaction tube, the catalyst bed will be as high as 60-200 cm and a temperature distribution will inevitably occur on account of the heat of reaction. The heat of reaction under consideration is 560 kcal/mol, which is considerably greater than that of the reaction involved in producing phthalic anhydride from orthoxylene or naphthalene. Theoretically, the heat of reaction generated in the production of PMDA from durene is no less than 1.8 times the heat of reaction generated in producing phthalic anhydride from orthoxylene. If durene is completely gasified to carbon dioxide or carbon monoxide, the amount of heat generated will even exceed 1,100 kcal/mol. Therefore, the reaction under consideration which will inherently generate more heat than other partial oxidation reactions has a tendency to create a broader temperature distribution in the catalyst bed. On top of this, if known catalysts are used, 35% or more conversion to gases will occur, which leads to an even broader temperature distribution in the catalyst bed that is out side the optimal temperature range for the catalyst. Then, a vicious cycle starts and an increased conversion to gases will reduce the yield of PMDA.
Various efforts have been made to remove the heat of reaction from the catalyst bed but from an engineering aspect of the reaction involved, the approaches that can be taken are limited and there has been no alternative to performing the reaction while sacrificing the production rate by reducing the concentration of durene feed, the height of the catalyst bed or the diameter of the reaction tube. In order to solve the aforementioned problems completely, it is important to develop a catalyst that has a high selectivity for PMDA with a small conversion to gases and which can be used over a broad optimal reaction temperature range. The use of such an ideal catalyst will facilitate temperature control in the catalyst bed and increase the efficiency of PMDA production.